A valve seat is a part that serves as a seat of an air valve or an exhaust valve, the part being connected to the valve and needed for maintaining air-tightness of a combustion chamber.
A valve seat is required to have, for example, the following functions: (1) a function of maintaining air-tightness in order to prevent leakage of compressed gas or combustion gas into a manifold; (2) a heat-conducting function for allowing heat in the valve to escape toward the cylinder head; (3) sufficient strength to withstand impact on the valve during seating; and (4) a wear-resistance function minimizing wear even in high-heat and high-load environments.
Additional required characteristics of the valve seat include: (5) lacking aggressiveness to an associated valve; (6) having a reasonable cost; and (7) being easy to scrape during processing.
An iron-based sintered alloy therefore is used in the valve seat so as to satisfy the functions and characteristics stated above.
For example, Patent Literature 1 discloses a valve seat in which an oxide formed mainly of triiron tetraoxide is formed on a surface and in an interior of an iron-based sintered alloy through oxidation treatment, and an area ratio of the oxide formed mainly of triiron tetraoxide is from 5% to 20% in a cross section of the iron-based sintered alloy in a state prior to installation on a cylinder head.